1. Field of the Invention
The present invention relates generally to an optical scanning apparatus and an image forming apparatus. More particularly, the present invention relates to a deflection mirror (vibration mirror) which is applicable to, for example, optical scanning apparatuses, light-scanning type display devices, and in-vehicle laser radar apparatuses, an optical scanning apparatus which includes such a deflection mirror, and an image forming apparatus, such as a digital copier, a printer, a plotter, and a facsimile that includes the above optical scanning apparatus.
2. Description of the Related Art
Conventionally known optical scanning apparatus deflects a light beam using a deflector such as a light deflecting unit, focuses the deflected light beam on a scanned surface as a minute spot-beam, and scans the scanned surface in a main scanning direction at a constant speed. Such an optical scanning apparatus is applied to various image forming apparatuses such as laser beam printers, laser beam plotters, facsimiles, and digital copiers, as a latent image writer. The optical scanning apparatus deflects and reflects a laser beam emitted from a laser light source using a light deflecting unit, for example, thereby scanning a scanned surface of an image carrier, for example. Simultaneously with the scanning, the optical scanning apparatus modulates the intensity of the laser beam (for example, by turning on and off the laser beam) according to image signals. Thus the optical scanning apparatus writes an image onto the scanned surface.
One type of widely used light deflecting units is a rotary polygon mirror (i.e., polygon scanner) that rotates at a constant speed. The rotary polygon mirror, however, is large, and its high-speed mechanical rotation generates vibrations which induce various inconveniences such as banding, temperature rise, noises, and increase in power consumption. Another light deflecting unit proposed is a micro mirror which has a resonant structure undergoing sinusoidal vibration based on a micro machine technology. When the micro mirror is employed as the deflector of the optical scanning apparatus, the optical scanning apparatus can be downsized and the above-listed inconveniences such as banding, temperature rise, noises, and increased power consumption caused by the vibrations can be significantly reduced.
In brief, when the micro mirror which undergoes sinusoidal vibration is employed in an image forming apparatus instead of the polygon mirror, a less-noisy, less power consuming, and environment-friendly image forming apparatus suitable for office environment can be provided.
When the micro mirror is employed as the deflector and undergoes sinusoidal vibrations, deflection angle varies in a sinusoidal manner. Currently, an f-theta lens is generally used in a writing optical system. However, when the micro mirror is employed as the deflector together with the f-theta lens in a scanning/imaging optical system, scanning speed slows near a peripheral image height and is not constant over the scanned surface. When the constant velocity characteristic of the scanning is degraded, the image becomes distorted at peripheral portions thereof in the main scanning direction, whereby the image quality lowers.
To deal with the problem described above, Japanese Patent Application Laid-Open No. 2005-215571 proposes an optical scanning apparatus including a scanning/imaging optical system whose imaging characteristic (farcsin characteristic) can be represented by a following equation (1):H=K×sin−1(φ/2φ0)  (1),where H represents image height, K represents proportionality factor, φ represents swing angle, and φ0 represents amplitude. Using the above scanning/imaging optical system, the optical scanning apparatus optically corrects a waist position of a main scanning light beam so as to realize a wide effective writing width and a favorable constant velocity characteristic of the scanning. In the disclosed optical scanning apparatus, however, when the optical correction is performed as described above, a spot diameter of the main scanning light beam on the scanned surface deviates more widely corresponding to the image height, thereby degrading the image quality.
In the optical scanning apparatus using the micro mirror undergoing the sinusoidal vibrations as a deflector, there is a trade-off between the constant velocity characteristic of the scanning and the deviation of the spot diameter of the main scanning light beam on the scanned surface corresponding to the image height. No optical scanning apparatus so far has realized favorable characteristics for both so as to realize high-quality image formation.
An optical scanning apparatus according to one embodiment described in Japanese Patent Application Laid-Open No. 2005-215571 decreases the deviation of the spot diameter of the main scanning light beam on the scanned surface corresponding to the image height, by using a scanning/imaging optical system which has the same level of sensitivity of a focusing position relative to an incident angle of a light beam to the deflector at a central image height and at a peripheral image height, in other words, which has the same scanning speed at a central image height and at a peripheral image height, or which makes the sensitivity (i.e., the scanning speed) lower (slower) at the peripheral image height than at the central image height. In other words, the optical scanning apparatus reduces the deviation in the spot diameter of the main scanning light beam on the scanned surface corresponding to the image height by using the scanning/imaging optical system whose linearity is substantially zero for all image heights on the scanned surface or whose linearity is biased towards a negative side at the peripheral image height. However, every embodiment disclosed in Japanese Patent Application Laid-Open No. 2005-215571 has problems as described below and has not succeeded in solving the problems mentioned above.
In a first embodiment of Japanese Patent Application Laid-Open No. 2005-215571, the deflector determines the width of the main scanning light beam. Therefore, the spot diameter of the main scanning light beam is larger at the peripheral image height than at the central image height on the scanned surface. The first embodiment corrects the deviation in the spot diameter of the main scanning light beam by giving the scanning/imaging optical system such a characteristic that the linearity is deteriorated, in other words, that the linearity is biased towards the negative side even further than the linearity of the apparatus employing an f-theta lens as the deflector. In the first embodiment, though the deviation of the spot diameter of the main scanning light beam corresponding to the image height is favorably corrected, the constancy of the scanning speed is significantly degraded because the linearity is made excessively negative even in comparison with the case where f-theta lens is employed as the deflector. Thus, high-quality images cannot be obtained.
In an optical scanning apparatus according to a second embodiment of the Japanese Patent Application Laid-Open No. 2005-215571, in which the deflector determines the width of the main scanning light beam, the f-theta lens is employed in the scanning/imaging optical system, and the swing angle of the micro mirror which serves as the deflector and undergoes the sinusoidal vibration is set smaller than maximum amplitude. When the swing angle of the micro mirror is small, linear changes become dominant over sinusoidal changes in the deflection speed variations. Thus, the scanning speed is made sufficiently constant, and the deviation in the spot diameter of the main scanning light beam corresponding to the image height is corrected to a favorable level. However, since the swing angle of the micro mirror is small, an effective writing width for image formation cannot be secured without a long distance between the micro mirror and the scanned surface. This requirement makes the optical scanning apparatus larger, limiting the overall dimension of the apparatus.
In an optical scanning apparatus according to a fourth embodiment of the Japanese Patent Application Laid-Open No. 2005-215571, which is similarly configured to an apparatus shown in FIG. 1, an f-theta lens is employed in the scanning/imaging optical system. In this case, the scanning speed slows at the peripheral image height as described earlier, and the scanning speed cannot be made constant over the scanned surface. When the constancy of the scanning speed is not sufficient, the image is sometimes distorted at the peripheral portion in the main scanning direction, degrading the image quality.
Further, in optical scanning apparatuses described in Japanese Patent Applications Laid-Open Nos. 2002-258204 and 2002-82303, a micro mirror which undergoes sinusoidal vibration is used as the deflector. The described optical scanning apparatuses do not optically correct the waist position of the main scanning light beam by using a scanning/imaging optical system which has the imaging characteristic as represented by the equation (1), but makes the swing angle of the micro mirror smaller relative to the maximum amplitude. As described above, in this case, the linear changes become dominant over sinusoidal changes in the deflection speed variations. Therefore, the scanning speed is made sufficiently constant, and the deviation of the spot diameter of the main scanning light beam according to image height is corrected to a favorable level. However, because of the reduced swing angle of the micro mirror, the optical scanning apparatus is required to be larger to secure an effective writing width necessary for image formation. Thus, there is a limitation on the overall dimension of the apparatus.
In order to realize a high image quality and downsizing of the optical scanning apparatus and the image forming apparatus, the optical scanning apparatus is required to realize constancy in the scanning speed on the scanned surface, and an acceptable level of deviation in the spot diameter of the main scanning light beam when scanning the scanned surface with the light beams deflected and reflected by the light deflecting unit. At the same time, the optical scanning apparatus is required to have a predetermined effective writing width necessary for image formation. These requirements need to be satisfied similarly by an optical scanning apparatus including a deflection mirror which utilizes resonance as mentioned earlier.
Conventionally, an optical scanning apparatus realizes a predetermined effective writing width and a favorable constancy in scanning speed by using a scanning/imaging optical system having an imaging characteristic as represented by the equation (1), and optically correcting the waist position of the main scanning light beam:H=K×sin−1(φ/2φ0)  (1),where H represents image height, K represents proportionality factor, φ represents swing angle, and φ0 represents amplitude. When the scanning/imaging optical system having the imaging characteristic represented by the equation (1) is employed, however, the spot diameter of the main scanning light beam becomes deviated corresponding to the image height as is known from Japanese Patent Application Laid-Open No. 2002-258204.
Further, when the scanning/imaging optical system which has imaging characteristics of the f-theta lens and which scans the scanned surface at a constant speed using the light beam deflected and reflected by a deflector such as a polygon mirror that rotates at a constant speed is employed in a writing optical system which uses a deflection mirror that undergoes sinusoidal vibration utilizing the resonance, the deviation in the spot diameter of the main scanning light beam on the scanned surface corresponding to the image height can be reduced while the predetermined effective writing width is secured. However, the linearity on the scanned surface is significantly biased toward the negative side, and the constancy of the scanning speed is deteriorated.
Further, when the swing angle of the deflection mirror which undergoes sinusoidal vibration utilizing the resonance is made small, though the constancy of the scanning speed and the deviation of the spot diameter of the main scanning light beam on the scanned surface corresponding to the image height can be made favorable, a larger distance between the deflection mirror and the scanned surface is required to secure a predetermined effective writing width, whereby the apparatus becomes large.